Established markets for solid-state image sensor technology include computer multimedia devices, video phones, toys, cameras, surveillance equipment, automotive, personal imaging equipment, x-ray imaging, manufacturing inspection, and telemedicine. In the camera market, in particular, solid-state image sensors which convert light into electric signals have been utilized to produce video cameras which are more lightweight, require less power, and are less expensive and more sensitive to light than traditional camera designs. Initial development strides in solid-state image sensor technology were directed toward silicon charge-coupled devices (CCD). A more recent trend in solid-state image sensor technology is the development of active pixel sensor (APS) technology.
APS technology has several important advantages over CCD technology. For example, the cost of fabricating an APS wafer is typically much less than the cost of fabricating a similar wafer using a specialized CCD process. Additionally, the APS wafer is produced with the same commercially available Complementary Metal-Oxide-Semiconductor (CMOS) device fabrication process used in nearly all modem microprocessors, memory, and logic chips. However, an essential aspect to the development of new, higher performance, ultra-low power APS image and hybrid sensor readout applications is improved spectral response and low thermal leakage in CMOS-based photosensor devices.
Presently, the photosensor device in Active Pixel Sensor (APS) imagers is either a photodiode or a Metal Oxide Semiconductor (MOS) capacitor (also known as a photo-capacitor). As compared with photo-capacitors, photodiodes are attractive because they are more sensitive to the blue end of the light spectrum and because they are less susceptible to the kind of oxide variations that cause threshold related fixed pattern noise. However, photodiodes tend to have greater thermal leakage than photo-capacitors and are more susceptible to the types of failures that cause white point defects. Since the causes of both leakage and point defects in diodes are related to junction depth and feature size, it is expected that photodiodes will eventually become too leaky for use in imager technology applications, especially as imager technology attempts to take advantage of a trend in CMOS technology toward finer line widths.
Accordingly, there is a need to provide a photosensor device with improved spectral response and lower thermal leakage than those devices found in the prior art.